1. Field of the Invention
The present invention relates to an image processing apparatus by which gradation processing with high color reproduction fidelity can be attained for a color image of an object.
2. Description of the Related Art
In an image processing apparatus for processing a color image, gradation processing and color correction processing are required so as to output, to a color printer, color signals read from an original using a reading device such as a color image scanner.
In color reproduction processing, for example, R, G, and B (R: red, G: green, and B: blue) signals from a scanner are converted into Y, M, and C (Y: yellow, M: magenta, and C: cyan) complementary color signals, as printer drive signals for original color signals. This processing is performed by a 3.times.3 matrix operation given by equation (1): ##EQU1##
On the other hand, an ordered dither method is generally used for the gradation processing. This method is simple, but undesirably impairs resolution of the resultant image.
An "error diffusion method" is known as a gradation processing method, which can provide higher resolution than the ordered dither method, and can also provide a sufficient gradation characteristic.
As U.S. Patent applications that disclose gradation processing methods using the error diffusion method, there are the following prior applications filed by the same applicants as for the present application:
(1) U.S. Ser. No. 756,630 filed on Sep. 9, 1991, and
(2) U S. Ser. No. 785,788 filed on Oct. 31, 1991.
All of the disclosures of these prior patent applications are incorporated in the present patent application.
In the "error diffusion method", a product of a digitization error of digitized surrounding pixels with a given weighting coefficient is added to a color signal of the target pixel, thus obtaining a digitized color signal of the target pixel by a fixed threshold value.
In the "error diffusion method", the digitization error generated upon digitization of the target pixel is diffused to the surrounding pixels to perform error compensation, thereby minimizing the digitization error. Therefore, when an input image is a photographic image, digitization processing can be performed to assure a satisfactory high gradation characteristic. However, in a color image, when the "error diffusion method" is independently applied to R, G, and B input color signals (three color signals), the three Y, M, and C digitized output signals cannot be in phase with each other. Thus, the digitized output signals having different phases are undesirably output, and a color image cannot be faithfully reproduced.